Raw coal is typically a composite of coal, pyritic sulfur and various other mineral matters. Even though the cost of separating coal from the pyritic sulfur and mineral matters is fairly expensive, in most cases, prior to use, coal is cleaned to reduce the amount of the foreign material present in it because of environmental factors, economic considerations, such as the cost of transporting noncombustible material over extended distances, and limitations on the amount of noncarbonaceous materials which can be tolerated in the process in which the coal is to be used.
Many techniques for cleaning coal have heretofore been proposed and a number of them are in current commercial use including air separation, jigging, froth flotation, cycloning and shaking on Diester tables. However, these techniques have disadvantages in that they are often inefficient and only coal particles in a relatively narrow size range can be handled.
Another known method for the separation of coal from mineral matter including pyritic sulfur involves milling or otherwise comminuting raw coal until it has been reduced to a particle size not exceeding 250 microns. Since coal is softer and easier to grind than pyritic sulfur and other mineral matter, the comminution of coal to a particle size of under 250 microns effects a partial release or separation of the coal from the mineral matter. The raw comminuted coal is then slurried in an aqueous liquid, typically clean water, and the comminution of the raw coal is continued until the raw coal has been subdivided into separate particles of coal and mineral matter including pyritic sulfur. After this comminution step has been completed, an agglomerating agent is added to the slurry and the slurry is agitated. The agitation is continued until the coal particles have separated from the particles of mineral matter and from the aqueous phase of the slurry and have coalesced into agglomerates of product coal. The product coal agglomerates are recovered from the slurry.
These conventional milling or comminuting processes are not fully satisfactory for various reasons, for example, the agglomerating agent is not efficiently recovered and reused in the process and there is no provision for removing and recycling oversized coal particles resulting from the comminution step prior to their being introduced into the agglomeration stage. The mineral and pyrite materials will not be fully liberated from such oversize particles and the mineral and pyrite materials will not be fully rejected by the agglomeration process. As a consequence, the quality of the product coal will not be upgraded as much as is desired. Additionally, the product agglomerates are typically recovered in a dry condition in which they are extremely friable and present a fire hazard due to the flammability of the dry coal particles, i.e., coal dust.
One process uses low levels (1.5-5% by wt. of coal) of an oil, such as fuel oil or spent crankcase oil, as the bridging liquid, and leaves it all in the product coal. The low level of bridging liquid used results in a very fine, floc-like, agglomerate product, collected either by screening or flotation. This material typically contains 50 percent moisture by weight. Such a high moisture level is unacceptable to a coal operator, so this wet floc product is dewatered to approximately 15 percent moisture by means of centrifuges. This product, which now has an acceptable moisture content, nonetheless is very fine, with all the attendant handling problems caused by that characteristic, and still contains the bridging oil, a potential source of odor and volatile organic air pollution.
Another potential pollution problem of the process practiced as described above is that since the product coal is very fine, coal is more likely to be lost in the tailing stream from the separation. The lower the oil level used, the greater this problem usually is. This coal, lost in the tailings, also contains some of the hydrocarbon bridging liquid, which can then be lost to the environment on disposal of the tailings. In the case of the use of recycled or waste oils, which often are contaminated with heavy metals, this can lead to significant pollution problems.
A dangerous, dry, pyrophoric product is produced when the process includes thermal recovery of the bridging liquid.
Accordingly, it is an object of the present invention to provide a process and apparatus for efficiently separating pyritic sulfur and other mineral matter from raw coal and agglomerating the coal particles to form agglomerates by the use of an azeotropically strippable bridging liquid and a binder.
It is a further object of the present invention to provide a process and apparatus for efficiently recovering a bridging liquid used in a coal particle agglomeration process by azeotropically stripping the bridging liquid from the agglomerated coal particles immediately after the agglomerating steps have been performed.
It is a still further object of the present invention to provide a process and apparatus for forming product coal agglomerates which are recovered in a moist condition and thereby exhibit reduced friability and flammability.
Other objects and purposes of this invention will be apparent to persons acquainted with processes of this general type upon reading the following specification and inspecting the accompanying drawings.